Cholesterol is essential for the growth and viability of higher organisms. It is a lipid that modulates the fluidity of eukaryotic membranes, and is the precursor to steroid hormones such as progesterone, testosterone, and the like. Cholesterol can be obtained from the diet, or synthesized internally in the liver and intestine. Cholesterol is transported in body fluids to tissues by lipoproteins, which are classified according to increasing density. For example, low density lipoprotein cholesterol (LDL) is responsible for transport of cholesterol to and from the liver and to peripheral tissue cells, where LDL receptors bind LDL, and mediate its entry into the cell.
Although cholesterol is essential to many biological processes in mammals, elevated serum levels of LDL cholesterol are undesirable, in that they are known to contribute to the formation of atherosclerotic plaques in arteries throughout the body, which may lead, for example, to the development of dyslipidemia and coronary artery diseases. Conversely, elevated levels of high density lipoprotein cholesterol (HDL-C) have been found, based upon human clinical data and animal model systems, to protect against development of coronary diseases. Low high density lipoprotein (HDL) is also a risk factor and marker for the development of metabolic syndrome and insulin resistance.
In general, excess cholesterol is removed from the body by a pathway involving HDL. Cholesterol is “effluxed” from cells by one of two processes—either by transfer to mature HDL, or an active transfer to apolipoprotein A-1 (Apo A-I). Transfer to mature HDL may involve both active and passive transfer mechanism. Transfer to Apo A-I and the generation of nascent HDL is mediated by ABCA1. In this process, lipid-poor HDL precursors acquire phospholipid and cholesterol forming nascent HDL which can then converted to mature HDL through the action of multiple plasma enzymes and the acquisition of cholesterol from peripheral tissues. HDL cholesterol is eventually transported to the liver where it is either recycled or excreted as bile. This process is often referred to as “reverse cholesterol transport”.
One method of treatment aimed at reducing the risk of formation of atherosclerotic plaques in arteries relates to modifying plasma lipid and lipoprotein levels to desirable levels. Such methods includes diet changes, and/or treatment with drugs such as derivatives of fibric acid (clofibrate, gemfibrozil, and fenofibrate), nicotinic acid, and HMG-CoA reductase inhibitors, such as mevinolin, mevastatin, pravastatin, simvastatin, fluvastatin, rosuvastatin, and lovastatin, which reduce plasma LDL cholesterol levels by either inhibiting the intracellular synthesis of cholesterol or inhibiting the uptake via LDL receptors. In addition, bile acid-binding resins, such as cholestyramine, colestipol and probucol decrease the level of LDL-cholesterol by reducing intestinal uptake and increasing the catabolism of LDL-cholesterol in the liver. Nicotinic acid through a poorly defined mechanism increase HDL levels and decreases triacylglycerol levels.
Another method of reducing the risk of formation of atherosclerotic plaques involves increasing the rate of cholesterol efflux from tissues and the formation of nascent HDL by increasing ABCA1 gene expression. The nuclear hormone receptor LXR is a key physiologic modulator of ABCA1 expression, and effectors of the LXR receptor may be used to pharmacologically increase ABCA1 activity. In addition to regulating ABCA1, LXR has been shown to at least partially regulate LXR target genes identified in macrophages, liver, intestine and other sites, which serve to orchestrate a concerted physiological response to excess sterol deposition. These include at least three other members of the ABC transporter family. Two of which have been identified as agents for another rare genetic disorder of sterol metabolism termed sitosterolemia. Another has been implicated as potential transporter of cellular cholesterol to mature and maturing HDL.
Unfortunately, systemic administration of potent full LXR ligands causes increased plasma triglycerides and liver lipid deposition due to the induction of several gene products involved in the synthesis of fats. Lipogenic genes in the liver are highly induced by LXR activation either directly, or via LXR induced transcription of the sterol regulatory protein SREBP1c. Selective LXR activation in macrophages, however, may have a protective role in reverse cholesterol transport while avoiding the pitfalls of inducing lipid bio-synthetic genes in the liver.
Additional advantages may be afforded by selectively interacting with the LXR enhancer/promoter transcription complex with LXR ligands that increase transcription of the subset of LXR target genes involved in cholesterol transport, but not the lipid bio-synthetic target genes. Tissue selective and/or unique partial LXR agonists may also provide the beneficial induction of ABCA1 (and other target genes) in macrophages and other non-hepatic tissues, while causing no or limited induction of SREBP1c and other lipogenic genes in the liver. See, for example Joseph S. B. and Tontonoz, P. (2003) Current Opinion in Pharmacology, 3:192-197 and Brewer H. B. et al. (2004) Arterioscler. Thromb. Vasc. Biol., 24:1755-1760.
It is desired to provide alternative therapies aimed at reducing the risk of formation of atherosclerotic plaques in arteries, especially in individuals deficient in the removal of cholesterol from artery walls via the HDL pathway. HDL cholesterol levels are a steady state measurement determined by the relative rates of HDL production and HDL clearance. Multiple enzymes and mechanisms contribute to both production and clearance. One method of increasing HDL levels would be to increase the expression of ABCA1 and the generation of nascent HDL resulting in increased HDL production. Accordingly, it is desired to provide compounds that are stimulators of the expression of ABCA1 in mammals both to increase cholesterol efflux and to raise HDL cholesterol levels in blood. This would be useful for the treatment of various disease states and dyslipidemias characterized by low HDL levels, such as coronary artery disease and metabolic syndrome.
It has also been shown that a combination of a drug that decreases LDL cholesterol levels and a drug that increases HDL cholesterol is beneficial; see, for example, Arterioscler., Thromb., Vasc. Biol. (2001), 21(8), 1320-1326, by Marian C. Cheung et al. Accordingly, it is also desired to provide a combination of a compound that stimulates the expression of ABCA1 with a compound that lowers LDL cholesterol levels.
It should be noted it has also been shown that raising production in macrophages locally reduces cholesterol deposition in coronary arteries without significantly raising plasma HDL cholesterol and without effecting cholesterol production by the liver. In this instance, raising ABCA1 expression is beneficial even in the absence of increased HDL cholesterol such that selective non-hepatic upregulation of ABCA1 may have beneficial effects on coronary artery disease in the absence of measurable effects on plasma lipid and lipoprotein levels.